I am wondering if anyone has any suggestions.
Cant open a 2017 file .... So,
You could just draw a 3D Box .... Height / width / Depth to suit the area of infill .... Make the Width and Height bigger than required, as you need to over lap the roof. (at this stage)
Copy your Roof and paste. (else where on the screen / or just save to a separate work file)
Explode the roof panels.
Click on a perspective View ... then using the X Y Z Toggles ...
"Edit Reference Point" >> Reference the Top plane on the bottom LHS corner of the 3D box
PLAN VIEW >> Place on the bottom LHS corner of the roof
PERSPECTIVE View >> (XYZ) ... Rotate the 3D box to the pitch of roof >> Now Lift it and clip it to the Roof corner.
PLAN VIEW >> Snaps >> Ortho >> Now slide the box so that it overlaps both planes of roof ... and shows a "Valley" to both sides of the existing roof.
TOOL >> 3D Slice >> Using reference lines ... and rotating the drawing >> Just Slice and Dice the 3D box.
Now that its Shaped to suit >>> Just "Edit Reference Point" the 3D box on a Fixed / clip-able point of the exploded roof >> and place it in your Roof Model.
The above will get you to what your after .. and further use of 3D tools will improve the End results.